There are several articles of clothing or accessories such as ski boats and helmets, fishing boots, boxing gloves, as well as swimming flippers which have to be adapted precisely to the parts of the body for which they are intended. The problem is further aggravated when the covering is rigid, as in the case of boots for downhill skiing, or helmets, because it is impossible in practice to mold a special covering adapted to the feet or to the head of each user. Now it is known that there are virtually never two individuals having the same morphology. This is why use is generally made of paddings made of deformable material in order to adapt the covering to the part of the body to be enclosed.
If the shoes are shaped from an entirely rigid shell, as are the majority of boots intended for downhill skiing the foot has to be held as closely as possible in the boot but still remain comfortable. Since the rigid shell is molded as a function of a given size and such a molding is inevitably standard, the boot is adapted to the foot by means of intermediate padding intended to match the shape of the foot as faithfully as possible.
Various solutions have already been proposed for forming this padding. Most of these techniques use a foamed plastic having the appearance of a sock placed in the rigid shell. This sock is shaped so as to reproduce the shape of the foot internally, fairly faithfully, and the foam allows it to be adapted to the slight morphological variations which exist between various feet of similar sizes. This solution is only partially satisfactory in so far as a standard sock is intended to adapt itself to feet of substantially the same size, that is to say the same length between the heel and the tip of the toe, but of which the configurations can, however, vary quite significantly among themselves. As a result, the foot is held fairly well but the comfort is uneven.
It has been proposed that this disadvantage be overcome by the in situ molding of expanded foam which is injected in a flexible chamber in the form of a sock, placed into the rigid shell and receiving the foot to which the boot is to be adjusted. The foam fills the free spaces between the foot and the shell, faithfully matching the shape of the foot. This is a solution which is capable of giving good results. Unfortunately, it is rather awkward to carry out the process, the foam is often distributed badly and the proportion of failures is high. It is also an expensive technique since each failure increases the price. Moreover this expanded foam does not have a very high resistance to fatigue and its density is fairly high. Although this solution was very successful initially, it has gradually been abandoned because of these factors.
It has also been proposed that cushions of wax or other similar materials be injected into certain regions to improve the retention of the foot in the shell. However, cushions of this type lack elasticity. It should also be noted that the present tendency is to produce light boots which have only a single fastener and not three or four, so that it is even more important to hold the foot in the shell as closely as possible, the gripping effect produced by fasteners being substantially reduced.
Use has already been made of thermo-formable foams produced from low density polyethylene which has been blown with nitrogen, in the medical sphere, particularly for hot-molding this foam around various parts of the body in order to produce cradles or orthopaedic supports or for adapting a prosthesis to a mutilated limb. The foam is heated to its thermo-forming temperature of about 140.degree. C. and applied to the part of the body to which this foam is to be adapted, the foam being modeled until it matches the desired shape perfectly. Contrary to any possible fears, there is virtually no risk of burning. In fact, bearing in mind that the density of the material is of the order of 0.04, that this is a foam with a low specific heat containing a large amount of gas and that the thermal conductivity of polyethylene is poor and that of the foam is even poorer owing to the large volume of gas which it contains, the amount of calories to be dissipated per unit time and per unit surface area is perfectly compatible with the heat exchange tolerable by the skin due to the circulation of blood. It should also be added to these considerations that the surface area of material in direct contact with the skin is very small relative to the surface area of skin covered by the foam.